This invention relates to a purification method for organometallic compounds, especially trimethylaluminum. More specifically, this invention is concerned with a method for lowering the concentration of an impurity, especially an organosilicon compound, which is contained in an organometallic compound such as trimethylaluminum effectively used for the production of a compound semiconductor and has a higher vapor pressure than the organometallic compound. This invention is also concerned with a purified organometallic compound obtained by the purification method.
Compound semiconductor materials such as gallium arsenide, indium phosphide and gallium phosphide are well known materials having uses in electronics industry in such applications as microwave oscillators, semiconductor light-emitting diodes, lasers and infrared detectors. The quality of a compound semiconductor available from an epitaxial growth of an organometallic compound is significantly controlled by impurities in the organometallic compound as a raw material. Therefor, high purity is required for the organometallic compound to obtain the compound semiconductor material with a high function.
Typical examples of organometallic compounds used in the production of compound semiconductors include trimethylgallium, trimethylindium, and trimethylaluminum. Among these, trimethylgallium and trimethylindium are produced from gallium chloride and trimethylaluminum and from indium chloride and trimethylaluminum, respectively, and the purities of trimethylgallium and trimethylindium are considered to depend on the purity of trimethylaluminum as their starting material. It is, accordingly, the most important step in the production of these organometallic compounds to highly purify trimethylaluminum.
The impurities in an organometallic compound to be purified include hydrocarbon compounds and in addition, organometallic compounds and metallic compounds containing metals different from the metal in the organometallic compound to be purified. Among these impurities, those having higher or similar vapor pressures compared with the organometallic compound to be purified may form impurity inclusions in a compound semiconductor to be produced from the purified organometallic compound if they still remain even after the purification, and therefore, are considered to be particularly harmful.
Among impurities having high vapor pressures, organosilicon components with Group IV metals contained therein are desired to be removed completely, because they lower electrical properties and optical properties of Group III-V compound semiconductors such as gallium arsenide if they are included in the compound semiconductors.
As processes for removing impurities having high vapor pressures, there have heretofore been reported adduct purification processes (JP-B 5-35154 and JP-A 62-185090), processes involving distillation in contact with metallic sodium or metallic potassium (JP-A 62-132888 and JP-A 8-12678), and processes for purifying liquid organometallic compounds by cooling and solidifying them (JP-A 2003-518007).
However, the adduct purification processes and the processes involving distillation in contact with metallic sodium or metallic potassium are accompanied by many drawbacks in that, as they require the addition of a solvent and the chemical for the treatment to an organometallic compound to be pured, the purity of the solvent and chemical need to be made very high before their addition, the solvent and chemical have to be treated after use, the solvent and chemical is costly, and the handling of the solvent and chemical is dangerous. The purification processes involving cooling and solidification of organometallic compounds also have problems in that applicable organometallic compounds are limited and the removal of impurities is insufficient.